Microstures And Mechanical Properties Of Mg-Al-Ca Alloys And Its Composites Influenced By Al/Ca Ratio

Mg-x Al-2Ca alloys with different Al/Ca ratios have been fabricated, the result showed that their secondary phase changed with the increase Al/Ca ratio from 1 to 4.5. Their corresponding relation of secondary phase and alloying components are: Mg-2Al-2Ca(AC22) which secondary phase are mainly Mg2 Ca and(Mg,Al)2Ca;Mg-3Al-2Ca(AC32) which secondary phase is mainly(Mg,Al)2Ca;Mg-5Al-2Ca(AC52) which secondary phase is mainly Al2Ca;Mg-9Al-2Ca(AC92) which secondary phase are mainly Al2 Ca and Mg17Al12. Correspondingly, 5mm 2vol.% Si Cp reinforced Mg-x Al-2Ca matrix composites have been fabricated by semisolid stirring casting technology.This article has investigated the effect of Al/Ca ratio on microstructure and mechanical property of extruded Mg-x Al-2Ca alloys and Si Cp/Mg-x Al-2Ca composites. The effect of thermal-stabilized Al2 Ca phase and thermal-instabilized Mg17Al12 phase on Mg-x Al-2Ca(x=5, 9) alloy and composites’ microstructure stability at elevated temperature have been discussed. Furthermore, the influence of Si Cp on Mg-x Al-2Ca matrix alloy’s microstructure, mechanical property and microstructure stability at elevated temperature have been studied. In view of the thermo-stabilized Al2 Ca phase existed in Mg-5Al-2Ca alloy and composite, elevated temperature tensile tests for them has been operated at temperatures from 290℃ to 350℃ and strain rates from 8.3 × 10-4 s-1 to 1.67 × 10-2 s-1. Constitutive equation for their hot tensile deformation has been established. Based on the constitutive equation, the deformation mechanism and behavior have been explored in this paper.The research results indicate that, the kind of secondary phase in Mg-x Al-2Ca alloys and composites is mainly depend on the Al/Ca ratio. With the increase of Al content, their secondary phase particles and Si Cp are tend to distribute along with the extruding direction. Besides, the distribution and amount of secondary phase particles would affect their DRX behavior, grain size and texture. For the Mg-x Al-2Ca matrix alloy, adding Si Cp could promote the precipitation of secondary phase and improve their distribution.After the heat treatment at 415℃, Si Cp/Mg-x Al-2Ca composites showed a better mechanical property than Mg-x Al-2Ca alloys. The Mg17Al12 has been solutionized in to the matrix, while Al2 Ca phase precipitated during heat treatment. Mg-9Al-2Ca alloy performed a better microstructure stability at elevated temperature than Mg-5Al-2Ca alloy. The growth of grain in Si Cp/Mg-x Al-2Ca has been impeded by Si Cp, which also lead to a beeter microstructure stability of Si Cp/Mg-x Al-2Ca composites at elevated temperature. After the heat treatment, the elongation of AC92 have been increased 63%,while 144% for Si Cp/AC92.The elevated temperature tensile tests on AC52 and Si Cp/AC52 revealed that their flow stress would increase with the increasing temperature or decreasing strain rate. Base on the hyperbolic sine function constitutive equation, activation energy for deformation Q and strain rate sensitivity exponents m have been calculated. The result shows that the diffusional creep and grain boundary sliding may occur for the AC52 alloy during hot deformation, while glide of dislocation might play a leading role for Si Cp/AC52.